Use flex-container on user dashboard (#27956)

Same as https://github.com/go-gitea/gitea/pull/26046 but for user
dashboard, the sidebar got a bit smaller and there is less padding
between sections.

<img width="1265" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/0c8d2faa-03ec-4515-a4f2-0a106ef2a928">

templates/user/dashboard/issues.tmpl

@@ -2,8 +2,8 @@
 <div role="main" aria-label="{{.Title}}" class="page-content dashboard issues">
 	{{template "user/dashboard/navbar" .}}
 	<div class="ui container">
-		<div class="ui stackable grid">
-			<div class="four wide column">
+		<div class="flex-container">
+			<div class="flex-container-nav">
 				<div class="ui secondary vertical filter menu gt-bg-transparent">
 					<a class="{{if eq .ViewType "your_repositories"}}active{{end}} item" href="{{.Link}}?type=your_repositories&repos=[{{range $.RepoIDs}}{{.}}%2C{{end}}]&sort={{$.SortType}}&state={{.State}}&q={{$.Keyword}}">
 						{{ctx.Locale.Tr "home.issues.in_your_repos"}}
@@ -59,7 +59,7 @@
 					{{end}}
 				</div>
 			</div>
-			<div class="twelve wide column content">
+			<div class="flex-container-main content">
 				<div class="list-header">
 					<div class="small-menu-items ui compact tiny menu list-header-toggle">
 						<a class="item{{if not .IsShowClosed}} active{{end}}" href="{{.Link}}?type={{$.ViewType}}&repos=[{{range $.RepoIDs}}{{.}}%2C{{end}}]&sort={{$.SortType}}&state=open&q={{$.Keyword}}">

templates/user/dashboard/milestones.tmpl

@@ -2,8 +2,8 @@
 <div role="main" aria-label="{{.Title}}" class="page-content dashboard issues repository milestones">
 	{{template "user/dashboard/navbar" .}}
 	<div class="ui container">
-		<div class="ui stackable grid">
-			<div class="four wide column">
+		<div class="flex-container">
+			<div class="flex-container-nav">
 				<div class="ui secondary vertical filter menu gt-bg-transparent">
 					<div class="item">
 						{{ctx.Locale.Tr "home.issues.in_your_repos"}}
@@ -33,7 +33,7 @@
 					{{end}}
 				</div>
 			</div>
-			<div class="twelve wide column content">
+			<div class="flex-container-main content">
 				<div class="list-header">
 					<div class="small-menu-items ui compact tiny menu list-header-toggle">
 						<a class="item{{if not .IsShowClosed}} active{{end}}" href="{{.Link}}?repos=[{{range $.RepoIDs}}{{.}}%2C{{end}}]&sort={{$.SortType}}&state=open&q={{$.Keyword}}">